Optical Technology in Current Measurement


Over the past 15 years, optical current sensors have received significant attention by a number or search groups around the world as next generation high voltage measurement devises, with a view to replacing iron-core current transformers in the electric power industry. Optical current sensors bring the significant advantages that they are non-conductive and lightweight, which can allow for much simpler insulation and mounting the designs. In addition, optical sensors do not exhibit hysteresis and provide a much larger dynamic range and frequency response than iron-core CT's.

A common theme of many of the optical current sensors is that they work on the principle of the Faraday effect. Current measurement plays an important role in protection and control of electric power systems. With the development of the conventional CT, the accuracy of the CT is up to 0.2% in the steady state power system. However many disadvantages of the conventional CT appear with the short circuit capacities of electric power systems getting larger and the voltage levels going higher for example, saturation under fault current conditions, ferroresonance effects, potential for catastrophic failure etc. Today there is number of interest in using optical current transformer (OCT) to measure the electric current by means of Faraday effect.

The benefits of an OCT are the inverse of the conventional CT's problems. That is, no saturation under fault current conditions, with out iron core and there fore no ferroresonance effects, with out oil and there fore cannot explode, light weight, small size, etc.

A common theme of many of the optical current sensors is that they work on the principle of the Faraday effect. Current flowing in a conductor induces a magnetic field, which, through the Faraday effect, rotates the plane of polarization of the light traveling in a sensing path encircling the conductor. Ampere's law guarantees that if the light is uniformly sensitive to magnetic field all along the sensing path, and the sensing path defines a closed loop, then the accumulated rotation of the plane of polarization of the light is directly proportional to the current flowing in the enclosed wire.

The sensor is insensitive to all externally generated magnetic fields such as those created by currents flowing in near by wires. A measurement of the polarization state rotation thus yields a measurement of the desired current. The technology originated 8 years ago to measure currents in Series Capacitor installations. Since then, it has been introduced not only to Series Capacitor and Thyristor Controlled Series Capacitor installations (FACTS), but also into High Voltage Direct Current Systems (HVDC).

These FACTS & HVDC systems gain their very high availability and reliability using the optically powered CT technology. Further integration of the optically powered technology has led to an economical and solid metering and protection current transformer without any of the known environmental problems associated with the oil or SF6-gas filled technology.

Researchers have perfected the OPCT to measure currents and transmit the data from high voltage system to ground potential using state of the art Laser technology. The fundamental of this technology includes the idea of using fiber optic cables to isolate the current transformers from ground potentials. The advantages of the optically powered scheme compared to the conventional, high voltage, free standing magnetic CT include an environmentally friendly, light weight, non seismic critical composite signal column together with proven, conventional, low voltage rated 'dry type' CT technology.